Power and thermal management issues are considerations in all segments of computer-based systems. While in the server domain, the cost of electricity drives the need for low power systems, in mobile systems battery life and thermal limitations make these issues relevant. Optimizing a system for maximum performance at minimum power consumption is usually done using the operating system (OS) or system software to control hardware elements. Most modern OS's use the Advanced Configuration and Power Interface (ACPI) standard, e.g., Rev. 3.0b, published Oct. 10, 2006, for optimizing the system in these areas. An ACPI implementation allows a processor core to be in different power-saving states (also termed low power or idle states), generally referred to as so-called C1 to Cn states. Similar package C-states exist for package-level power savings but are not OS-visible.
When a core is active, it runs at a so-called C0 state, and when the core is idle, it may be placed in a core low power state, a so-called core non-zero C-state. The core C1 state represents the low power state that has the least power savings but can be entered and exited almost immediately, while an extended deep-low power state (e.g., C3) represents a power state where the static power consumption is negligible, but the time to enter/exit this state and respond to activity (i.e., back to C0) is longer.
In addition to power-saving states, performance states or so-called P-states are also provided in ACPI. These performance states may allow control of performance-power levels while a core is in an active state (C0). In general, multiple P-states may be available, namely from P0-PN. In general, the ACPI P-state control algorithm is to optimize power consumption without impacting performance. The state corresponding to P0 may operate the core at a maximum voltage and frequency combination for the core, while each P-state, e.g., P1-PN, operates the core at different voltage and/or frequency combinations. In this way, a balance of performance and power consumption can occur when the processor is active based on utilization of the processor. While different P-states can be used during an active mode, there is no ability for independent P-states for different cores operating at different voltages and frequencies of a multi-core processor, and accordingly, optimal power savings cannot be attained while achieving a desired performance level, since at best all active cores may be able to operate at different frequencies but they all must share the same voltage.